Skeletal muscle fiber types: fast vs slow-twitch

Speed of contraction

The different types of skeletal muscle fibers exhibit variations in their contraction speed, primarily due to the expression of distinct isoforms and quantities of myosin ATPase present. Myosin ATPase is located in the head region of myosin and plays a crucial role in catalyzing ATP hydrolysis, which provides the necessary energy for muscle contraction. The activity of myosin ATPase is positively correlated with the speed of sarcomere contraction. Type II muscle fibers contain a higher amount of myosin ATPase and, consequently, contract more rapidly compared to type I muscle fibers.

Additionally, metabolism is another factor that influences the speed of muscle contraction. Anaerobic metabolism, which is predominantly associated with type IIx muscle fibers, is linked to faster contraction. This will be further elaborated in the following paragraph.

Aerobic vs. anaerobic metabolism

The last part of aerobic respiration, oxidative phosphorylation, occurs in mitochondria and is dependent on oxygen. Oxygen is delivered to the muscle fibers by capillaries and is stored by a molecule similar to hemoglobin, called myoglobin. Myoglobin is necessary for constant oxygen supply to myocytes, which consume much more energy than other tissues in the human body. Mitochondria and myoglobin are responsible for the red color of muscle tissue. Aerobic respiration produces larger amounts of ATP than anaerobic metabolism

Anaerobic glycolysis can be performed in the absence of oxygen. It produces ATP almost 100 times faster than oxidative phosphorylation, but only two ATP molecules per glucose molecule can be produced. The production of lactate as a byproduct of anaerobic metabolism leads to metabolic acidosis, resulting in muscle fatigability.

Type I: slow oxidative fibers

Type I (SO) muscle fibers (a.k.a. ‘slow twitch’ fibers) are primarily characterized by their reliance on aerobic metabolism. They contain a large number of mitochondria and therefore are capable of sustained contractions over an extended period without being easily fatigued due to the large amounts of ATP they can produce. However, due to their relatively small diameter, they are not well-suited for generating high levels of tension. Type I fibers are well supplied by capillary networks (relative to their size) which supply oxygen and also possess large amounts of myoglobin which stores oxygen within the fibers themselves; this gives these fiber types a reddish color in fresh specimens. As mentioned, due to the relatively low amounts of myosin ATPase, type I fibers have a slow contractile speed. They are the first type of fibers to be recruited during contraction.

As a result, these fibers are more suitable for activities that involve low-level tension and endurance. They are found in a higher proportion in skeletal muscles responsible for the maintenance of posture (e.g. erector spinae group, soleus muscle) and are the first fibers to be recruited during a muscle contraction. Additionally, type I fibers are often utilized during low-intensity, prolonged aerobic exercises like walking, or during endurance sports.

Type IIa: fast oxidative fibers

Type IIa (FO) muscle fibers, also known as intermediate fibers, can be considered as a transitional type between type I (SO) and type IIb (FG) muscle fibers; they are larger and generally more numerous than type I fibers. They primarily rely on aerobic metabolism but are capable of producing ATP at a faster rate than type I fibers (via anaerobic metabolism if necessary). While they are resistant to fatigue and can sustain contractions for a prolonged period, their ability to do so is less than type I. However, they are capable of generating faster contractions and higher tension compared to type I fibers but less that of type IIx fibers.

Type IIa fibers usually possess a similar density of mitochondria as type I fibers as well as moderate amounts of glycosomes which store glycogen/glycolytic enzymes required for anaerobic respiration. However, they have lower myoglobin content and capillary density relative to their size, resulting in a lighter pink coloration. These fiber types are recruited after type I but before type IIx muscle fibers.

These characteristics contribute to their intermediate properties. Type IIa (FO) fibers are particularly useful for prolonged movements that require more tension than what type I fibers can generate, such as running and swimming. They provide the necessary endurance and moderate force production for these activities.

Type IIx: fast glycolytic fibers

Type IIx (FG) muscle fibers primarily rely on anaerobic metabolism (i.e. glycolysis) for energy production. Due to their metabolic profile, they are capable of executing fast and high-intensity contractions. However, they also fatigue more quickly compared to other muscle fiber types.
Type IIb muscle fibers, mistakenly used as synonyms for type IIx muscle fibers, are fast-twitch muscle fibers which primarily rely on glycolysis. Type IIb and IIx fibers mainly differ in the isoform of myosin ATPase. The type IIb muscle fibers are not usually found in human muscle tissue.

Type IIx fibers possess a substantial glycogen depot, which allows for the rapid release of glucose and energy production to facilitate rapid contractions. Morphologically, they have a larger diameter and appear whiter in color. This is attributed to their limited reliance on oxidative phosphorylation and subsequent lower density of mitochondria, capillary structures and myoglobin content. Type IIx fibers are primarily mobilized for quick and forceful movements i.e. activities that involve short bursts of anaerobic effort, such as weightlifting, sprinting and jumping, predominantly recruit these muscle fibers. They are crucial for generating rapid and powerful muscular contractions.